X-ray tubes are used in a variety of industrial and medical applications. For example, X-ray tubes are employed in medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and material analysis. Regardless of the application, most X-ray tubes operate in a similar fashion. X-rays, which are high frequency electromagnetic radiation, are produced in X-ray tubes by applying an electrical current to a cathode to cause electrons to be emitted from the cathode by thermionic emission. The electrons accelerate towards and then impinge upon an anode. The distance between the cathode and the anode is generally known as A-C spacing or throw distance. When the electrons impinge upon the anode, the electrons can collide with the anode to produce X-rays. The area on the anode in which the electrons collide is generally known as a focal spot.
X-rays can be produced through at least two mechanisms that can occur during the collision of the electrons with the anode. A first X-ray producing mechanism is referred to as X-ray fluorescence or characteristic X-ray generation. X-ray fluorescence occurs when an electron colliding with material of the anode has sufficient energy to knock an orbital electron of the anode out of an inner electron shell. Other electrons of the anode in outer electron shells fill the vacancy left in the inner electron shell. As a result of the electron of the anode moving from the outer electron shell to the inner electron shell, X-rays of a particular frequency are produced. A second X-ray producing mechanism is referred to as Bremsstrahlung. In Bremsstrahlung, electrons emitted from the cathode decelerate when deflected by nuclei of the anode. The decelerating electrons lose kinetic energy and thereby produce X-rays. The X-rays produced in Bremsstrahlung have a spectrum of frequencies. The X-rays produced through either Bremsstrahlung or X-ray fluorescence may then exit the X-ray tube to be utilized in one or more of the above-mentioned applications.
In certain applications, it may be beneficial to lengthen the throw length of an X-ray tube. The throw length is the distance from cathode electron emitter to the anode surface. For example, a long throw length may result in decreased back ion bombardment and evaporation of anode materials back onto the cathode. While X-ray tubes with long throw lengths may be beneficial in certain applications, a long throw length can also present difficulties. For example, as a throw length is lengthened, the electrons that accelerate towards an anode through the throw length tend to become less laminar resulting in an unacceptable focal spot on the anode. Also affected is the ability to properly focus and/or position the electron beam towards the anode target, again resulting in a less than desirable focal spot—either in terms of size, shape and/or position. When a focal spot is unacceptable, it may be difficult to produce useful X-ray images.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.